The pressure at which typical gas distribution systems supply gas may vary according to the demands placed on the system, the climate, the source of supply, and/or other factors. However, most end-user facilities equipped with gas appliances such as furnaces, ovens, etc., require the gas to be delivered in accordance with a predetermined pressure, and at or below a maximum capacity of a gas regulator. Therefore, gas regulators are implemented into these distribution systems to ensure that the delivered gas meets the requirements of the end-user facilities. Conventional gas regulators generally include a closed-loop control actuator for sensing and controlling the pressure of the delivered gas.
In addition to a closed loop control, some conventional gas regulators include a balanced trim to improve the reaction of the gas regulator to variations in the downstream pressure. The balanced trim is adapted to reduce the influence of the upstream pressure on the performance of the gas regulator. The upstream pressure is placed in fluid communication with a balancing diaphragm to apply a force to the control element of the gas regulator in the opposite direction as the force of the downstream pressure. Accordingly, as the upstream pressure varies, a corresponding force is applied to balance the force created by the upstream pressure as described further below so that the gas regulator acts in response to the downstream pressure only.
Some conventional gas regulators also include secondary monitoring devices (overpressure protection devices), such as overpressure monitoring devices, slam shut devices, token alarms and the like, that perform a responsive action if a sensed input pressure, such as a pressure downstream of the regulator, varies from a predetermined normal operating pressure range. An overpressure protection device controls the pressure downstream of the regulator in the event that the regulator fails, thereby allowing the downstream pressure to increase to undesired levels. In the event the regulator fails and the downstream pressure rises above a predetermined monitor setpoint pressure, the overpressure protection device operates to close the valve port of the regulator valve and cut off the flow of gas to the downstream components of the gas distribution system. As demand increases and/or the problem with the regulator is resolved and the downstream pressure drops, the overpressure protection device opens the valve port and thereby allows gas flow downstream.
Typically, an overpressure protection device, such as an overpressure monitoring device, responds in the event that the downstream pressure (i.e., pressure in the outlet) exceeds a cutoff pressure that is sensed by a diaphragm. Specifically, a bottom surface of the diaphragm of the overpressure monitor partially defines a control cavity such that pressure in the control cavity acts on the diaphragm. In one embodiment of an overpressure monitoring device, a control line extends from the control cavity to a portion of the outlet of the regulator valve, and the control line is disposed external to a both a housing of the overpressure monitoring device and portion of the regulator valve defining the valve outlet. So configured, downstream pressure is accurately communicated to the control cavity. However, such a configuration may be expensive to install and maintain.
Alternatively, overpressure protection devices may include an internal sense tube that extends from the control cavity to the outlet of the regulator valve and that is disposed through internal portions of the housing of the overpressure protection device and a portion of the body of the regulator valve defining the valve outlet. The internal sense tube may have a generally cylindrical shape having a constant cross-section. The internal sense tube may also have an open terminal end that extends into the regulator valve outlet, and the pressure of the fluid flowing within the valve at or adjacent to the outlet is sensed through the open terminal end of the internal sense tube. A portion of the internal sense tube that includes the terminal end may extend obliquely into the outlet or may extend parallel to the general direction of fluid flow through the outlet. However, as flow velocities and/or flow demand changes, the pressure communicated to the control cavity by the internal sense tube may correspondingly vary. Typically, a pressure lower than the actual control pressure is communicated to the control cavity via the open terminal end of the internal sense tube, thereby artificially driving the valve to induce boost in regulating pressures. More specifically, as flow passes over the end of the internal sense tube, a low pressure zone is created at the open terminal end which results in the registered pressure being lower than the actual pressure. As a result, the accuracy of the pressure measurement changes as the flow rate changes, thereby negatively impacting the ability of the overpressure protection device to protect the downstream system.